Technologies for inspecting the inside of an inspection target based on an X-ray projected image in a non-destructive manner are technologies that are essential in a medical field, an industrial nondestructive inspection field, and the like. Particularly, an X-ray image sensor that directly takes in an X-ray projected image as electronic data is widely used owing to a speedy imaging process, image reading support using image processing, applicability to a moving image, and the like. As such an X-ray image sensor, a device called flat panel detector (FPD) is mainly used. This is one type of metal-oxide semiconductor (MOS) type image sensor. In the FPD, in each of pixels that are two-dimensionally arranged, a photoelectric conversion section converting an X ray into electric charge and a switching device taking out signal electric charge accumulated in the photoelectric conversion section to the outside are arranged. The FPD is prepared on a substrate formed of glass having a large area or the like by using a thin film semiconductor technology. The reason for this is that a reduction optical system capable of responding to the X ray cannot be easily produced, and thus, the size of the FPD needs to be equal to or more than that of an inspection target. Thus, as a switching device arranged in each pixel, a thin film transistor (TFT) is used.
Regarding the FPD, as systems converting an X ray into electric charge, there are two systems when largely divided. One is an indirect conversion system converting an X ray into visible light and converting the converted visible light into electric charge. The other one is a direct conversion system directly converting an X ray into electric charge. A conventional example of the FPD of the indirect conversion system is disclosed in FIG. 5 of Japanese Patent Application Laid-Open No. 4-206573. This FPD has a structure in which a phosphor layer is stacked on a formation part of a photodiode and a transistor through an insulating film. According to the radiation of an X ray, the phosphor layer emits visible light, and the emitted visible light is converted into electric charge using the photodiode. In Japanese Patent Application Laid-Open No. 4-206573, an example is disclosed in which a photodiode and a transistor are formed using amorphous silicon (a-Si). On the other hand, a conventional example of an FPD of the direct conversion type is disclosed in FIG. 1 of Japanese Patent Application Laid-Open No. 11-211832. This FPD has a structure in which pixels each having a photoconductive layer connected to a transistor are formed on a substrate. An X ray is absorbed by the photoconductive layer and is directly converted into electric charge. Japanese Patent Application Laid-Open No. 11-211832 discloses a method using ZnO, CdS, CdSe, or the like as a photoconductive layer. In FPDs of the two systems described above, a signal is output as electric charge, is converted into a voltage by a signal detecting circuit such as an integrator disposed outside, and is formed as a digital signal. The FPD represented here outputs signal electric charge acquired by each pixel, and the signal electric charge is not amplified within the pixel. Thus, there is a possibility that the FPD is classified as a passive-pixel sensor (PPS).
Recently, in a medical field, an X-ray diagnostic apparatus is strongly requested to provide low radiation exposure and high definition. In a case where an X ray radiation amount is decreased for low radiation exposure, signal electric charge detected by the FPD is decreased, whereby the S/N ratio is degraded. In addition, in a case where the pixel size of the FPD is decreased for high definition, signal electric charge is decreased according to a decrease in the pixel size, whereby the S/N ratio is degraded. In other words, in order to achieve both the low radiation exposure and the high definition, it is essential to raise the S/N ratio of the FPD.
As a technology for realizing a high S/N ratio in an image sensor, there is a technology called active-pixel sensor (APS) that is applied to a CMOS image sensor. In this technology, in each pixel of an image sensor, an amplifier circuit is provided in addition to a photoelectric conversion device such as a photodiode, a signal of the photoelectric conversion device is amplified, and the amplified signal is output. According to this technology, high definition of an image sensor can be realized without degrading the S/N ratio of a signal. Generally, while a CMOS image sensor is integrated on a single-crystal Si substrate and is mainly used for an optical camera and the like, there are attempts for applying such an APS technology to a thin film semiconductor. For example, there is a method disclosed in Japanese Patent Application Laid-Open No. 1-184954.
However, in order to apply the APS to the thin film semiconductor, a means that corrects a threshold voltage variation of the TFT is essential. For example, in a case where polycrystalline Si TFT is used as a thin film semiconductor, there is a problem in that in-plane variations of the threshold voltage are very large. These variations are intrinsic problems due to variations in the grain size of polycrystalline Si and the like. On the other hand, in a case where a-Si TFT is used for the amplification of a signal, owing to an amorphous structure, a problem of variations of the threshold voltage due to the crystal structure of a polycrystalline Si TFT or the like does not occur. However, a problem relating to the reliability occurs. The problem is that the threshold voltage varies much in a case where a voltage is continued to be applied between the gate and the source of an a-Si TFT. In a TFT used for an amplifier circuit, a voltage causing the TFT to be constantly in a conductive state is continuously applied between the gate and the source. For this reason, a threshold voltage of the TFT for an amplifier circuit varies, and an output voltage varies according to the variation of the threshold voltage as well. This problem relating to the reliability similarly occurs also in TFTs using amorphous oxide semiconductor.
As a means that corrects output variations of pixels accompanied with such threshold variations of TFTs, several methods have been proposed.
One method is disclosed in Japanese Patent Application Laid-Open No. 10-108075. According to this method, before an image sensor starts to operate, a reference voltage is supplied to an amplification TFT of each pixel. Then, output variations in the case of supplying the reference voltage are maintained in a memory or the like, and a variation component is eliminated from a detected signal at the time of the operation. However, in this method, there is a problem in that the dynamic range of a detection circuit detecting a signal of an image sensor needs to be set to be very large. For example, in a case where a polycrystalline Si is used as the thin film semiconductor, a threshold voltage variation of the TFT may be 1 V or more. In a case where it is considered that the amplitude of the output voltage of an image sensor is about 1 V, this means that the dynamic range of the detection circuit needs to be set to be twice or more. To set the dynamic range twice or more with the precision and the operating speed of the detection circuit being maintained makes it difficult to design the detection circuit, which leads to an increase in the manufacturing cost.
In another method, a technology called correlated double sampling (CDS) is used. According to this technology, an offset voltage error of the amplifier circuit is eliminated by taking a difference between an output voltage including a signal component of an image sensor and an output voltage after resetting the photodiode. An example in which this technology is applied to an MOS-type image sensor is illustrated in FIG. 1. FIG. 1 is a circuit diagram that illustrates a circuit 200 corresponding to one pixel and a signal processing circuit 600 performing CDS. The pixel 200 is one pixel of an image sensor and is configured by a photodiode 210, an amplification transistor 220, a selection transistor 230, and a reset transistor 240. A source terminal of the selection transistor 230 is connected to a signal line Dm. A load resistor 310 is connected to each signal line. In a case where the selection transistor 230 is in a conductive state, a source follower circuit is configured by the amplification transistor 220 and the load resistor 310. The signal processing circuit 600 is configured by an initial-stage amplifier 610, a switch 620, a switch 621, a capacitor 630, a capacitor 631, and a differential amplifier 611.
The operation of the CDS will be described using a timing diagram illustrated in FIG. 2. At time t0, a selection signal Gn becomes a high level, and the selection transistor 230 becomes a conductive state. Accordingly, the electric potential of the source of the amplification transistor 220 is output as the electric potential of both ends of the load resistor 310 disposed at the signal line Dm. At time t1, a control signal φ1 becomes the high level, and the switch 620 becomes a conductive state. At time t2, the control signal φ1 changes to a low level, and the switch 620 becomes a non-conductive state. Accordingly, a voltage V1 of the load resistor 310 at time t2 is maintained at the capacitor 630. At time t3, a reset control signal Rn becomes the high level, and the reset transistor 240 becomes the conductive state. Accordingly, the electric potential of a cathode of the photodiode 210 is reset to VBP. At time t4, a control signal φ2 becomes the high level, and the switch 621 becomes the conductive state. At time t5, the control signal φ2 is changed to a low level, and the switch 621 becomes a non-conductive state. Accordingly, the voltage V1 of the load resistor 310 at time t5 is maintained at the capacitor 631.
Here, it is assumed that the voltage amplification factor of each of the initial-stage amplifier 610 and the differential amplifier 611 is one. In a case where the cathode voltage Vpc of the photodiode 210 at time t2 is Vsig, the voltage V1 becomes α·Vsig−Vof, and this voltage is maintained at the capacitor 630. At time t5, since the cathode voltage of the photodiode 210 is reset to VBP, the voltage V1 becomes α·VBP−Vof. Since this voltage is maintained at the capacitor 631, the output V4 of the differential amplifier 611 becomes α(Vsig−VBP). Here, a is a voltage amplification factor of the source follower circuit that is configured by the amplification transistor 220 and the load resistor 310, and Vof is an offset voltage. The offset voltage Vof is a value that depends on the threshold voltage of the amplification transistor 220. In a case where the threshold voltages of the amplification transistors 220 of pixels vary, the offset voltages Vof vary as well. However, in a case where a CDS operation is performed, the offset voltage is not included in the output voltage of the differential amplifier 611, and thus, the influence of variations of the threshold voltages of the amplification transistors 220 can be eliminated.
However, in the image sensor illustrated in FIG. 1, a problem of decreasing the light sensitivity occurs. In a case where the CDS is performed, an output corresponding to an optical signal and an output corresponding to the reset voltage of the photodiode are required. For this reason, a selection signal used for selecting a pixel, and a reset control signal used for resetting the photodiode are required. Such signal lines need to be provided for each pixel row, and a space used for laying the signal lines is necessary. In a case where such signal lines are laid, a fill factor that is a ratio of the area of a photodiode to the area of a pixel is decreased. As a result, the light sensitivity decreases. In addition, a drive circuit used for supplying a selection signal and a reset control signal is required for each pixel row, and the manufacturing cost of the image sensor is high.
In an image sensor including APS pixels, a method of performing a reset operation without using a signal dedicated for reset control is disclosed in FIG. 2 of Japanese Patent Application Laid-Open No. 10-108074. The method disclosed here is a method in which sensor resetting is used for a next selection signal. In this method, a dedicated reset wiring does not need to be laid, and the fill factor is not decreased.